Stupid AB bias question

[torrence]

This has been bugging me for a while now, so let me just ask a stupid question. With a class AB push-pull amp, I was under the impression that fixed bias was highly preferred to cathode bias, mainly to avoid cut-off distortion. Since cathode bias is really a kind of local feedback it makes sense to me that this would cause trouble when a tube is turned off.

Looking around at various PP designs, however, I notice many class AB amps running with cathode bias (including the tubelab simple PP). So is the issue of cutoff distortion greatly exaggerated, does it depend how far the operating point is from pure class A, or are the other benefits of cathode bias worth a little added distortion?

[kavermei]

Hi,

cathode bias is not feedback when the cathode resistors are properly 'decoupled' with a large enough capacitor. If the capacitor is not present, then yes, you have local feedback.

Cathode bias is notorious for lacking bass just because that capacitor is sometimes choosen too small.

Cathode bias has the advantage of needing one less voltage supply, and being more tolerant of mismatched tubes. Fixed bias has the advantage of not wasting plate voltage in the cathode circuit, and it's easier to experiment with different bias poins (just turning a pot versus swapping out resistors).

HTH,
Kenneth

[torrence]

Hi Kenneth,

Thanks for the reply. You list all the arguments I am aware of, but even if you have a large cathode capacitor it seems like the Vk bias point will still vary depending on the average RMS cathode current (which can change considerably in AB depending on the input signal level) and this will still tend to give a kind of negative feedback or compression (less gain when the amp is playing loud). With a larger capacitor it seems like this would persist for longer when the amp goes from a loud to soft.

I guess it all depends on the timescales, and I should just try this out on the bench and see how it sounds.

Regards

[Eli Duttman]

Dude,

You are forgetting about the effect loop NFB has. Make the open loop gain sufficiently large and the closed loop gain is solely a function of the values in the gain set pair.

Long term reproducible results are 1 of the best reasons to use loop NFB.